The number of Americans who believe global warming is occurring has declined to its lowest since 1997, though at 72 percent, it's still a broad majority. The drop has steepened in the last year-and-a-half – almost exclusively among conservatives and Republicans.

This ABC News/Washington Post poll also finds that support for government action to address the issue, while still a majority, likewise is down from its levels in summer 2008.

Belief that Earth is warming peaked at 85 percent in 2006, then flattened before turning back. Even with the decline, Americans who think global warming probably is occurring outnumber those who think not by nearly 3-1, 72 percent to 26 percent.

Levels of concern are undiminished among those who think it is happening, and intensity of sentiment has risen: Eighty-two percent call it a serious problem right now (it was a similar 84 percent last year); 44 percent call it "very serious," up 6 points.

On policy, 76 percent now favor unspecified government action on global warming, down from 86 percent in summer 2008. This now includes 55 percent who favor the United States taking steps even if countries such as China and India do less; that's down from 68 percent.

On one specific proposal, 53 percent support a cap-and-trade system to limit greenhouse gases. That's slipped from 59 percent in summer 2008.

Discussion of government action to address global warming has intensified in advance of a United Nations-sponsored conference in Copenhagen Dec. 7-18. Representatives of 191 countries have been invited; the White House is expected to say shortly whether President Obama will go.

GETTING WARMER? – Belief that global warming is occurring – specifically, that the world's temperature has been going up slowly in the past hundred years – was 76 percent in an Ohio State University poll in 1997 and 85 percent in an ABC/Time/Stanford University poll in spring 2006. It subsided to 80 percent last year, vs. 72 percent now.

The ideological and partisan nature of the change, especially in the last year, supports previous research finding that views on global warming are heavily informed by political and ideological predispositions. (So, for example, are views of the economy, particularly when its condition or direction aren't clear.)

Since summer 2008, belief that warming is occurring fell by 13 points among conservatives while holding essentially steady among liberals and moderates. It fell by 20 points among Republicans and 8 points among independents while steady among Democrats. Grouping Republicans with independents who lean toward the Republican Party finds a 17-point drop in this group, compared with no real change (-1) among Democrats and Democratic-leaning independents.

Republicans, Conservatives Oppose Policies to Reduce Global Warming

The changes in the two groups in which it's chiefly occurred are striking ones. Last year leaned Republicans by 72-25 percent believed the Earth was warming; today it's 55-43 percent. Conservatives last year divided by 69-28 percent on the question; today, by contrast, it's 56-41 percent. Combining these groups – that is, among conservative Republicans – a bare majority now says global warming is not occurring, the only group in which more than half says so.

Looking back another year, to spring 2007, shows changes that also are disproportionately among conservatives and Republicans. In this comparison, belief that global warming is occurring has dropped by 21 points among conservatives vs. 7 points and 5 points, respectively, among liberals and moderates; and by 18 points among leaned Republicans vs. 7 points among leaned Democrats.

WHY? – Policy preferences could hold a clue as to why these changes have occurred. Conservatives and Republicans broadly oppose proposed government measures to deal with climate change. A heightened sense that such changes may be coming, particularly since the Obama administration took office, may encourage more people in these groups to express disbelief that global warming is occurring in the first place.

Data in this survey show, as expected, that belief that global warming is occurring predicts support for measures to deal with it. But the reverse also is true: Views on government measures to address climate change predict belief in whether it's occurring. Directionality is difficult to establish, and may well run both ways. In any case, including one of these variables when predicting the other in a statistical model increases the model's explanatory power.

In further evidence, belief that global warming is occurring has fallen since summer 2008 entirely among people who oppose cap and trade and who oppose unilateral action by the United States. Among their policy opposites, belief has held steady.

The change in views among conservatives and Republicans has occurred even as scientific consensus and the urgency of warnings about the impact of a warmer Earth have increased. Previous research, however, has shown that conservatives and Republicans simply are less disposed to accept those warnings as reliable.

SERIOUSLY? – Among people who believe climate change is occurring, there again are ideological and partisan differences on its seriousness. Three-quarters of conservatives in this group say it's a serious problem now, 9 and 13 points fewer than the number of moderates and liberals who say so. The partisan gap is wider: Among Republicans who think it's happening, 63 percent call it serious; that jumps to 82 percent of independents and 90 percent of Democrats.

Cap and Trade Program to Reduce Global Warming

ACTION/BELIEF – As noted, belief that global warming is occurring, and that it's a serious problem, are the strongest independent predictors of support for government action in general, and a cap-and-trade law in particular.

Among people who say it's happening and is a serious problem now, 73 percent favor unilateral action by the United States to address global warming; that drops to 40 percent among those who think it's happening but isn't serious, and just 24 percent of those who don't think it's occurring. Similarly, support for cap and trade peaks at 65 percent of those who see a serious problem now, drops to 42 percent among those who think Earth is warming but don't see a serious problem at this point and bottoms out at 33 percent of those who don't think it's occurring.

Support for action, like belief in the phenomenon and its seriousness, also are influenced by ideology and partisanship. Seventy-two percent of liberals and 67 percent of moderates favor action by the United States even if other countries do less; that dives to 34 percent among conservatives. And conservatives are more than 20 points less apt to back cap and trade.

Politically, Democrats and Republicans are at odds on both of these, with independents closer to Republicans, particularly on cap and trade.

In terms of change the past year-and-a-half, support for cap and trade has declined by 12 points among Republicans and Republican-leaning independents, vs. 4 points among leaned Democrats; it's also declined more among conservatives than among liberals and moderates. Support for unilateral action, though, has fallen more generally across these groups; it's down by 15 points among leaned Democrats as well as by 13 points among leaned Republicans.

There are other factors in views on policy. People who think the economy is recovering are 20 points more apt to support cap and trade, a significantly predictive factor even when controlled for other variables, including partisanship and ideology. (Economic views don't significantly predict opinions on unilateral U.S. action in general.) Additionally, younger adults are 16 points more apt than their elders (and 25 points more likely than seniors) to support unilateral U.S. action. This holds as an independent predictor when controlled for other factors.

OTHER DATA – Other recent polls have shown similar declines in belief that global warming is occurring, with results differing in degree given the different questions posed. They also show less credence among conservatives and Republicans, but with changes involving other groups as well as these, unlike the ABC/Post results.

Gallup, Fox and Pew Polls on Global Warming

A Gallup poll in March found an 8-point decline from 2008, to 53 percent, in belief that the effects of global warming "have already begun" to happen; a 6-point decline, to 60 percent, in personal worry about it; and a 6-point rise, to 41 percent, in the belief its seriousness is "generally exaggerated," a view Gallup called "somewhat volatile" in polls since 2001. (Views that the effects have begun fell by 16 points among conservatives from March 2008 to March 2009, compared with 6 points among moderates and an insignificant 1-point gain among liberals in Gallup's data.)

In a Fox News poll last May, 69 percent (of registered voters) said they "believe global warming exists," down from 82 percent in January 2007. And a Pew Research poll last month found a 14-point drop, from 71 percent in spring 2008 to 57 percent, in people saying there's "solid evidence" temperatures have been rising the past few decades. The ABC/Post question asks if people think temperatures probably have or probably have not been rising, a lower bar than "solid evidence."

METHODOLOGY – This ABC News/Washington Post poll was conducted by telephone Nov. 12-15, 2009, among a random national sample of 1,001 adults, including landline and cell-phone-only respondents. Results for the full sample have a 3.5-point error margin. Click here for a detailed description of sampling error. Sampling, data collection and tabulation by TNS of Horsham, Pa.